Different structures of two Cu(I) complexes constructed by bridging 2,2-(1,4-butanediyl)bis-1,3-benzoxazole ligand: Syntheses, structures and properties

Article abstract of DOI:10.1016/j.ica.2017.10.023

Reaction of 2,2-(1,4-butanediyl)bis-1,3-benzoxazole (BBO) ligand with [Cu(CH₃CN)₂(PPh₃)₂][X] (X = ClO₄, PF₆) afforded a copper(I) coordination polymer (CP) {[Cu(BBO)(PPh₃)]. ClO₄}_∞ (1) and a binuclear complex [Cu₂(BBO)(PPh₃)₄]. 2PF₆. 2CH₂Cl₂ (2) (where PPh₃ = triphenylphosphine). Two complexes have been characterized. The structural analysis revealed that in complexes 1–2, all Cu(I) ions are tri-coordinated and the geometric structure around the central Cu(I) atom can be described as planar trigonal configuration. Complex 1 exhibits a one-dimensional coordination polymer by two BBO bridging adjacent copper(I) ions and extending along the b axis, forming a single-stranded helix chain structure that extends into 2-D layer frameworks through π^…π interactions. Complex 2 shows a binuclear structure and the unit extends to a 2D supramolecular layered framework through C-H^…F interactions. Moreover, compared with emissive bands of the free ligand in the solid state, the photoluminescent transition of the Cu(I) complexes 1–2 may be attributed to metal-to-ligand charge-transfer [MLCT].

Full text of DOI:10.1016/j.ica.2017.10.023

Inorganica Chimica Acta 471 (2018) 17–22
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Inorganica Chimica Acta
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Research paper
Different structures of two Cu(I) complexes constructed by bridging
2,2-(1,4-butanediyl)bis-1,3-benzoxazole ligand: Syntheses, structures
and properties
⇑
Yuling Xu, Shanshan Mao, Kesheng Shen, Xinkui Shi, Huilu Wu , Xia Tang
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2017
Received in revised form 8 October 2017
Accepted 23 October 2017
Reaction of 2,2-(1,4-butanediyl)bis-1,3-benzoxazole (BBO) ligand with [Cu(CH3CN)2(PPh3)2][X] (X = ClO4,
PF6) afforded a copper(I) coordination polymer (CP) {[Cu(BBO)(PPh3)].ClO4} (1) and a binuclear complex
1
.
[Cu2(BBO)(PPh3)4].2PF6 2CH2Cl2 (2) (where PPh3 = triphenylphosphine). Two complexes have been char-
acterized. The structural analysis revealed that in complexes 1–2, all Cu(I) ions are tri-coordinated and
the geometric structure around the central Cu(I) atom can be described as planar trigonal configuration.
Complex 1 exhibits a one-dimensional coordination polymer by two BBO bridging adjacent copper(I) ions
and extending along the b axis, forming a single-stranded helix chain structure that extends into 2-D
layer frameworks through p. . .p interactions. Complex 2 shows a binuclear structure and the unit extends
to a 2D supramolecular layered framework through C-H. . .F interactions. Moreover, compared with emis-
sive bands of the free ligand in the solid state, the photoluminescent transition of the Cu(I) complexes
1–2 may be attributed to metal-to-ligand charge-transfer [MLCT].
Keywords:
Copper(I) complex
Photoluminescent
Crystal structures
N-containing heterocyclic
Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction
Copper(I), with a d10 electronic configuration, is well known for
its tetrahedral coordination geometry, which is linked by two or
Benzoxazole is a five-membered ring compound containing
both nitrogen and oxygen fused to a benzene ring and the benzox-
azole ring is one of the most common heterocyclics in medicinal
chemistry [1]. The binding force of metal ions with N and O atoms
is different. In general, N-donor azole groups have shown strong
coordination ability to the first-row transition metal ions [2]. P-
containing ligands have large steric hindrance and rigid structure,
which is beneficial to reduce the structural deformation of Cu(I)
complexes, and the unique electronic effect of the P atoms can sta-
bilize the copper(I) [3]. Accordingly, in order to enhance the phos-
phorescence intensity of copper(I) complexes, the usually-used N-
containing heterocyclic ligands and phosphine ligands are simulta-
neously coordinated [4]. N-containing heterocyclic ligands, due to
the diversity of coordination modes and configurations, are typi-
cally used as neutral ligands in the synthesis of complexes [5].
The counterions are introduced to balance the charge when study-
ing the neutral ligand, which not only affect the coordination envi-
ronment of the metal ions, but also the overall structure of the
complex [6].
four bridging ligands, leading to regular 1D chains, 2D layers, or
3D frameworks. Examples of the Cu(I) atoms adopting a 3-coordi-
nate trigonal environment [7], afforded unprecedented structural
motifs, such as 3D frameworks with rectangular extended channels
or interwoven honeycombs. The observations clearly indicate the
important role of coordination preferences in the determination
of the architectures. The construction of copper(I) complexes con-
taining triphenylphosphine and heterocyclic nitrogen ligands has
drawn much attention in recent years, due to their interesting opti-
cal properties [8], as well as potentially-widespread applications in
materials science, catalysis, synthetic and medicinal chemistry [9].
The selection of N-heterocyclic chelating ligand is paramount
because, with it, copper(I) complexes possess high stability to pro-
mote luminescence from metal-to-ligand charge transfer (MLCT)
and long fluorescence lifetime [10].
Helical structures have attracted an intense interest in coordi-
nation chemistry not only for their ubiquitous appearance in nat-
ure, a typical example being the DNA molecule, but also for their
practical implications in multidisciplinary areas, such as structural
biology, optical devices and biomimetic chemistry [11]. Until now,
many single-, double-, triple-, and even multiple-stranded helices
as well as circular and cylindrical helices have been prepared and
comprehensively discussed. To get such helices, the crucial step
⇑
Corresponding author.
E-mail address: wuhl@mail.lzjtu.cn (H. Wu).
https://doi.org/10.1016/j.ica.2017.10.023
0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

18
Y. Xu et al. / Inorganica Chimica Acta 471 (2018) 17–22
is to choose multifunctional organic ligands containing appropriate
coordination sites linked by a proper spacer with specific posi-
tional orientation.
m
/cmꢀ1): 3054 (w), 1558 (m), 1479 (w), 1434 (w), 1246 (m), 1095
(w), 837 (s), 746 (m), 694 (m), 513 (w).
On the basis of the above considerations, in this paper, a N-
heterocyclic chelating bisbenzoxazole ligand combined with the
second ligand triphenylphosphine and copper(I) salt was employed
to obtain two copper(I) complexes. The structures of the two com-
plexes have been characterized by elemental analysis, IR and X-ray
single crystal diffraction. In addition, the luminescence and ther-
mal stabilities properties have been studied.
2.3. X-ray crystallography
Crystallographic data of complexes 1–2 were collected on a Bru-
ker Smart CCD diffractometer with graphite-monochromated Mo-
Ka radiation (k = 0.71073 Å) at 296 K. Data reduction and cell
refinement were performed using the SMART and SAINT programs
[12]. The absorption corrections are carried out by the empirical
method. The crystal structures of 1–2 were solved by direct meth-
ods and refined by full-matrix least-squares against F2 of data
using SHELXTL program [13]. The crystals of compound 1 are sus-
ceptible to weathering, resulting in the disorder about the uncoor-
dinated ClOꢀ4 ion and a high wR2 value. All non-hydrogen atoms
were refined by using anisotropic thermal parameters. All hydro-
gen atoms were included in the calculated position and refined
with the isotropic thermal parameters riding on the parent atoms.
Crystal parameters and details of the final refinement parameters
are shown in Table 1. The selected bond lengths and bond angles
for complexes 1–2 are listed in Table 2.
2. Experimental section
2.1. Materials and general methods
All the chemicals and solvents were reagent grade and were
used without further purification. 1H NMR spectra were recorded
on a Varian VR400 MHz spectrometer with TMS as an internal
standard. Melting points were detected on an X-4 digital micro
melting-point apparatus. The C, H and N elemental analyses were
determined using a Carlo Erba 1106 elemental analyzer. The IR
spectra were recorded in the 4000–400 cmꢀ1 region with a Nicolet
FT-VER-TEX 70 spectrometer using KBr pellets. Fluorescence spec-
tral data were obtained on a 970-CRT fluorescence spectropho-
tometer at room temperature. Thermogravimetric analysis (TGA)
was carried out on METTLER TOLEDO TGA1 thermal analyzer from
room temperature to 800 °C with a heating rate of 10 °C/min under
N2 atmosphere.
3. Results and discussion
3.1. Characterization of the complexes
The synthetic routes to Cu(I) complexes are shown in Scheme 1.
The elemental analysis of complexes 1–2 is in good agreement
with the theoretical compositions. The free ligand is soluble in
organic solvents but insoluble in water. The Cu(I) complexes are
remarkably soluble in polar aprotic solvents such as DMF, DMSO,
dichloromethane and acetonitrile, slightly soluble in ethanol and
methanol, and insoluble in water, diethyl ether, petroleum ether
and hexane.
2.2. Syntheses of the ligand and complexes
2.2.1. Synthesis of BBO
O-aminophenol (8.73 g, 0.08 mol), adipic acid (5.85 g, 0.04 mol)
and 140 mL of polyphosphoric acid (PPA) were mixed and trans-
ferred into a 500 mL three-necked bottle. The mixture was stirred
at 200 °C for 6 h. Thereafter, the resulting solution was cooled to
120 °C and poured into ice-water and 10% NaOH solution was used
to adjust its pH to 10 after which a lot of pink precipitation was
collected by filtration, washed with water and dried under vac-
uum. The crude product was recrystallized from ethanol to obtain
white solids of BBO. Yield: 89%, m.p: 118–120 °C. 1H NMR
(400 MHz, CDCl3) d: 7.65–7.67 (m, 2H, Ph-H), 7.44–7.47 (m, 2H,
Ph-H), 7.65–7.67 (m, 4H, Ph-H), 2.99–3.03 (m, 4H, CH2), 2.04–
2.07 (m, 4H, CH2). Anal. Calcd (%) for C18H16N2O2: C 73.95, H
3.2. X-ray structures of the complexes
3.2.1. Crystal structure of 1
Single-crystal X-ray diffraction analysis reveals that the asym-
metric unit consists of one copper(I) ion, one BBO ligand, one triph-
Table 1
Crystal and structure refinement data for complexes 1–2.
Complex
1
C
2
Empirical formula
Molecular weight
Crystal system
Space group
a (Å)
36H31ClCuN2O6P
C92H80Cl4Cu2F12N2O2P6
1928.28
Monoclinic
P2(1)/c
10.6838(6)
22.7066(13)
19.0226(11)
90
103.1610(10)
90
4493.5(4)
2
1.425
0.773
1972
23634
8359
0.0236
1.019
0.0375
5.52, N 9.58. Found: C 73.87, H 5.46, N 9.49. IR (KBr pellet,
m/
717.59
Triclinic
P-1
cmꢀ1): 3051 (w), 2947 (w), 1609 (m), 1572 (s), 1453 (s), 1381
(m), 1246 (s), 943 (m), 831 (w), 746 (s).
10.627(6)
10.696(6)
16.733(13)
91.775(14)
93.851(13)
119.079(9)
1654.2(18)
2
b (Å)
c (Å)
2.2.2. Preparation of complexes
a
(°)
Two complexes were prepared using a similar procedure. CH2-
Cl2 solution (12 mL) of BBO (58.4 mg, 0.2 mmol) and copper(I) salt
(0.1 mmol: [Cu(CH3CN)2(PPh3)2]ClO4, 77 mg; [Cu(CH3CN)2(PPh3)2]
PF6, 81.5 mg) were stirred at ambient temperature for 4 h to give
a colorless solution. The solvent was then removed at reduced
pressure. The resultant residue was again dissolved in 5 mL CH2Cl2,
and there was a slow diffusion of hexane into the above solution.
Colorless crystals suitable for X-ray diffraction studies were
obtained after 5 days.
b (°)
c
(°)
V (Å3)
Z
Dcalcd (g/cm3)
1.44
0.839
l
(mmꢀ1
)
F(0 0 0)
740
Total reflections
Unique reflections
Rint
8161
5518
0.0434
1.032
GOF on F2
Complex 1. (72% yield). Anal. Calcd (%) for C36H31ClCuN2O6P: C
R1 (I > 2
wR2 (I > 2
R1 (all data)
r
(I))
0.0769
0.2044
0.1465
0.2614
1.195
60.3, H 4.35, N 3.90. Found: C 59.8, H 4.11, N 3.78. IR (KBr pellet, m/
r(I))
0.0922
0.0518
0.0997
0.444
cmꢀ1): 3054 (w), 1554 (m), 1434 (w), 1246 (m), 1090 (s), 747 (m),
695 (m), 505 (w).
wR2 (all data)
D
D
q
q
max, eÅꢀ3
min, eÅꢀ3
Complex 2. (80% yield). Anal. Calcd (%) for C46H40Cl2CuF6NOP3:
C 57.3, H 4.18, N 1.45. Found: C 57.26, H 4.15, N 1.41. IR (KBr pellet,
ꢀ0.835
ꢀ0.670

Y. Xu et al. / Inorganica Chimica Acta 471 (2018) 17–22
19
Table 2
weaker interaction of copper(I) with perchlorate. The two types
of BBO bridged two adjacent copper(I) ions alternately, resulting
Selected bond distances (Å) and angles (°) for complexes 1–2.
in a ‘‘1” shape single-stranded helix chain (Fig. 2a). In 1, the Cu. . .
-
Parameter
Value
Parameter
Value
Cu separations across the BBO bridges are 9.7609 and 9.5067 Å,
respectively. Hinging at metal ions, the pitch of the helix is 10.81
Ǻ, corresponding to the length of b-axis. Furthermore, adjacent
copper(I) chains are connected together by weak p. . .p interactions
(3.68 Å) [17] from the benzoxazole rings to generate a 2D layer
supramolecular framework (Fig. 2b). Further insight into this
supramolecular interaction discloses that each 2D layer can be
simplified as puckered (6,3) network like interwoven honeycombs,
as shown in Fig. S1.
1
Cu(1)-N(1)
2.024(6)
1.997(6)
2.193(2)
N(2)-Cu(1)-N(1)
N(2)-Cu(1)-P(1)
N(1)-Cu(1)-P(1)
104.3(2)
128.32(17)
127.13(17)
Cu(1)-N(2)
Cu(1)-P(1)
2
Cu(1)-N(1)
Cu(1)-P(1)
Cu(1)-P(2)
2.0382(18)
2.2719(6)
2.2630(7)
N(1)-Cu(1)-P(2)
N(1)-Cu(1)-P(1)
P(2)-Cu(1)-P(1)
115.97(6)
112.33(6)
129.28(2)
3.2.2. Crystal structure of 2
The crystal structure of complex 2 consists of a binuclear [Cu2-
BBO(PPh3)4]2+ motif, two hexafluorophosphate anions and two
guest dichloromethane molecules per formula unit (Fig. 3). The
coordination environment around the copper(I) center is best por-
trayed as NP2 planar trigonal (total angle around the copper(I) ion:
357.58°), ligated by one nitrogen atom (N1) from BBO and two
phosphine atoms (P1, P2) from two distinct PPh3. The bond lengths
are Cu1-N1 = 2.038(9), Cu1-P1 = 2.2719(6) and Cu1-P2 = 2.2630(7)
Ǻ, respectively, which, to the best of our knowledge, are similar to
the corresponding values for reported Cu(I) complexes [18]. Inter-
estingly, the Cu-P distance of 2.2719(6) Å and 2.2630(7) Ǻ in 2 is
longer than that in 1 (2.193(2) Å); this may be due to the coordina-
tion of each copper(I) atom by two bulky triphenylphosphine. The
deviation of the copper(I) ion from the N1-P1-P2 plane is 0.1968 Å,
which is larger than the value of 0.0542 Å for 1. This might relate to
the stronger interaction of copper(I) with hexafluorophosphate
(Cu. . .F (PF6ꢀ) = 3.0467 Å). As shown in Fig. S2, in complex 2, the
Cu. . .Cu separations across the BBO bridges are 9.0259 Å, which
are shorter than the values for complex 1, which might relate to
the different interactions of copper(I) with counterions.
Scheme 1. Syntheses of complexes 1–2.
enylphosphine molecule as well as one ClOꢀ4 counterion in 1
(Fig. 1). The Cu(I) ion is tri-coordinated by two nitrogen atoms
(N1, N2) from two distinct BBO ligands and one phosphine atom
(P1) from triphenylphosphine ligand in a planar trigonal environ-
ment (total angle around the copper(I) ion in 1: 359.75°). The
Cu–N bond lengths range from 1.997(6) to 2.024(6) Å, while the
Cu-P bond lengths are 2.193(2) Å, which are in the normal range
[14]. The distance of Cu1-N1 is longer than Cu1-N2, which may
be due to the presence of p. . .p interactions between BBO ligands.
The Cu(I) ion from the N1-N2-P1 plane is 0.0542 Å, indicating that
the Cu1, N1, N2, and P1 are approximately coplanar. This is smaller
to the reported values for [Cu(PPh3)(H2CPz’2)](ClO4) (0.186 Å) [15]
and [Cu(L1”)(PPh3)](ClO4) (0.170 Å) [16], that might relate to the
It is notable that complex 2 unit extends to a 2D supramolecular
layered framework through intermolecular CAH. . .
F hydrogen
bonding (2.4536–2.5561 Å) between hexafluorophosphate anions,
[Cu2BBO(PPh3)4]2+ motif and CH2Cl2, as shown in Fig. S3, which
are comparable to those reported in analogous complexes contain-
ing CAH. . .F hydrogen bonding [19], with two dichloromethane
molecules filled inside each unit. Further insight into this
Fig. 1. ORTEP diagram of 1 with ellipsoids shown at the 30% probability level. (The hydrogen atoms and counterions are omitted for clarity).

20
Y. Xu et al. / Inorganica Chimica Acta 471 (2018) 17–22
Fig. 2. (a) Side view of the 1D supramolecular meso-helical chain (The other groups of BBO, counterions and PPh3 are omitted for clarity); (b) 2D framework built by p. . .
interactions in complex 1. (Hydrogen atoms, counterions and PPh3 are omitted for clarity).
p
Fig. 3. ORTEP diagram of 2 with ellipsoids shown at the 30% probability level. (The hydrogen atoms, counterion and solvent molecule are omitted for clarity).
supramolecular interaction discloses that each 2D layer can be
simplified as a wave-like network, as shown in Fig. S4.
From the above research content, it can also be found that the
same bridging ligand with different copper(I) salt formed the dif-
ferent structures of the complexes. The counterion anions might
have a significant influence on the structures of the complexes.
[20]. The –C@N bands are shifted by 18 cmꢀ1 in the Cu(I) com-
plexes, which suggests the participation of benzoxazole nitrogen
in coordination with copper(I) ion. Similar shifts also appeared in
complexes 2, which give the same conclusion. The weaker peaks
at 505 and 513 cmꢀ1 for complexes 1–2, respectively, may be
stretching vibrations of Cu-P bonds. The perchlorate complex 1
exhibits a strong band at 1090 cmꢀ1, suggesting the stretching
vibration of non-coordinated ClOꢀ4 anion in the complex. However,
a strong band exhibited at 837 cmꢀ1 is consistent with the pres-
ence of PFꢀ6 anion in the complex. This fact agrees with the result
determined by X-ray diffraction.
3.3. Thermal properties
Thermogravimetric analysis was carried out between 30 and
800 °C in a static atmosphere of nitrogen to investigate the thermal
stabilities of complexes 1–2 as depicted in Fig. S5. The decomposi-
tion of the complexes 1–2 starts at 171–193 °C and completes at
563–656 °C. In summary, the TGA illustrates that the complexes
1–2 possess good thermal stability, thus rendering them as poten-
tial candidates for practical applications.
3.5. Fluorescence properties
The photoluminescent properties of complexes 1–2 and corre-
sponding free ligand have been investigated at room temperature
in the solid state. As shown in Fig. 4, upon excitation at 347 nm,
the free ligand displays a blue-green emission peak at 422 nm,
3.4. IR spectra
which may be attributed to
p-
p⁄ transition [21]. Whereas, upon
The IR spectra of the free ligand BBO and its Cu(I) complexes
were compared. Free BBO shows strong bands at 1246 cmꢀ1 and
the excitation of the complexes 1–2 sample at 350 nm, two emis-
sion peaks were observed at about 404–406 nm and 422 nm. Com-
pared with the free BBO ligand, the former peaks may be attributed
to metal-to-ligand charge-transfer [MLCT] [10] while the latter
1572 cmꢀ1, with the two bands being attributed to the
m
(CAO)
and
m(C@N) frequencies of the benzoxazole group, respectively

Y. Xu et al. / Inorganica Chimica Acta 471 (2018) 17–22
21
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In summary, one bisbenzoxazole open-chain ligand and its cop-
per(I) complexes have been synthesized and characterized. In com-
plexes 1–2, the bisbenzoxazole ligand adopts the l2-bridging mode
to link two copper(I) atoms, forming two different copper(I) motifs.
Complex 1 exhibits a one-dimensional coordination polymer,
while 2 shows a binuclear complex, revealing that different coun-
terions have a significant influence on the structures of the com-
plexes. In the complexes, the coordination geometry around the
central Cu(I) atom is planar trigonal. Moreover, complexes 1–2
exhibit acceptable thermal stability and good photoluminescence
properties in solid states at ambient temperature. The investiga-
tion puts forward these copper(I) complexes as new promising
luminescence materials for future applications.
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The present research was supported by the National Natural
Science Foundation of China (Grant No. 21367017), Foundation of
A Hundred Youth Talents Training Program of Lanzhou Jiaotong
University (Grant No. 152022) and the Science and Technology
Program of Gansu Province (Grant No. 17JR5RA090).
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.ica.2017.10.023.
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